I have paid the fee and received the "Your Odoo Enterprise subscription is active" e-mail, along with the enterprise code. However, their are no instructions or links to documentation that tells me where I should add the code or how I should install Odoo 9 Enterprise from scratch, if that is what is required!

This is the first time I have paid for a software licence and seemingly have no information on how to install the software!

I have the Community version installed on Centos 7 using the nightly mirror. How do I change the Community version of Odoo to the Enterprise version?

Technically, the Enterprise edition (EE) is a series of additional modules, so you can easily add those modules and change the addons path of your Community edition (CE) install to use them (the EE web addon is a complete replacement for the CE web addon).